World's Biggest Virus May Have Ancient Roots

Pandoraviruses were discovered lurking in the mud of Chile and Australia, half a world apart.

courtesy of Chantal Abergel and Jean-Michel Claverie

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Originally published on July 20, 2013 5:14 am

Researchers have discovered the largest virus ever, and they've given it a terrifying name: Pandoravirus.

In mythology, opening Pandora's Box released evil into the world. But there's no need to panic. This new family of virus lives underwater and doesn't pose a major threat to human health.

"This is not going to cause any kind of widespread and acute illness or epidemic or anything," says Eugene Koonin, an evolutionary biologist at the National Institutes of Health who specializes in viruses.

Instead, the Pandoravirus opens up a host of questions about the origins of life on Earth, according to its discoverer, Jean-Michel Claverie of Aix-Marseille University in France. He says, "We believe that those new Pandoraviruses have emerged from an ancestral cellular type that no longer exists."

A typical virus is a tiny sack of genetic material that injects itself into a much larger cell and uses it to make more viruses. The Pandoravirus is enormous by comparison—large enough to be seen in an ordinary microscope (about 1 micrometer).

It's so big it's hard to even tell it's a virus, Claverie says. "They don't have a regular shape like a regular viruses, they really look like blobs. And so they really look like small bacteria".

Bacteria are single-celled organisms that reproduce on their own. Claverie first stumbled across giant viruses a decade ago, when another researcher brought him one that was misidentified as a bacterium. It was only when Claverie saw it infect amoebas that he realized it was a virus.

After a recent survey found genetic hints of giant viruses in seawater, Claverie and his team decided to go on the hunt. They teamed up with oceanographers and scooped out sediment samples from the coast of Chile and a freshwater pond in Australia.

They brought back the samples and placed them in a solution filled with antibiotics, to kill any bacteria that might have been along for the ride. Then they exposed the samples to their laboratory amoebas.

"If they die, we suspect that there's something in there that killed them," says Chantal Abergel, Claverie's co-author and also his wife.

It worked. The infected amoebas spawned lots of Pandoraviruses. When Abergel and Claverie sequenced the genome of the new virus, they were in for a shock. Its genetic code is roughly twice the size of the record-holding Megavirus. And it seems almost completely unlike anything else on the planet. Only 6 percent of its genes resembled the genes other organisms. Claverie says he thinks the Pandoraviruses may come from a different origin – perhaps radically different.

"We believe that those new Pandoraviruses have emerged from a new ancestral cellular type that no longer exists," he says. That life could have even come from another planet, like Mars. "At this point we cannot actually disprove or disregard this type of extreme scenario," he says.

But how did this odd cellular form turn into a virus? Abergel says it may have evolved as a survival strategy as modern cells took over. "On Earth it was winners and it was losers, and the losers could have escaped death by going through parasitism and then infect the winner," she says.

Eugene Koonin, who wasn't involved in the research, isn't buying this theory. "These viruses, unusual as they might be, are still related to other smaller viruses," he says.

The virus's size is probably part of its survival strategy. Amoebas and other simple creatures could mistake it for bacteria and try to eat it, opening them up to infection. "The internal environment of the amoeba cell provides a very good playground for acquiring various kinds of genes from different sources," Koonin says. He thinks that the Pandoravirus's unusual genome may be a mishmash of random genetic material it's sucked up from its hosts.

Nevertheless, Koonin says, the new virus is fascinating. And he predicts this is only the beginning. "We are going to see many, many more giant viruses discovered around the world, some of which, probably will be bigger than Pandoraviruses."

Meanwhile, Claverie is also looking at what Pandoravirus actually does in the wild. The fact that it can be found on different continents, and in both fresh water and salt, suggests it may be a big player in underwater ecosystems around the globe.

Copyright 2013 NPR. To see more, visit http://www.npr.org/.

Transcript

AUDIE CORNISH, HOST:

From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish.

MELISSA BLOCK, HOST:

And I'm Melissa Block. Today in the journal Science, researchers announced the discovery of the largest virus ever seen and they gave it a name that sounds like something out of a sci-fi thriller. So should we be worried? To answer that question, here's NPR's Geoff Brumfiel.

GEOFF BRUMFIEL, BYLINE: There are a lot of scary viruses running around these days, so when scientists discover a big virus and call it Pandoravirus, you've just got to ask. This is not some giant virus that will go rampaging through the cities, knocking over buildings?

EUGENE KOONIN: No, no, no. We can be completely, completely relaxed about such prospects. They do not exist.

BRUMFIEL: Eugene Koonin is a researcher at the National Institutes of Health who studies viruses.

KOONIN: This is not going to cause any kind of widespread and acute illness or epidemic or anything, never ever.

BRUMFIEL: But Koonin is still interested in the new virus because it's freakishly bit. Is it as big as a housefly?

KOONIN: No. But you can definitely see it very well with a regular microscope.

BRUMFIEL: And that's huge in the virus world. Remember, a normal virus is tiny. It's just a sack of genetic material that injects itself into a cell. It can't reproduce on its own, so it hijacks the cell's machinery to churn out more viruses. The Pandoravirus is so big, it's hard to tell it's even a virus, says Jean-Michel Claverie, the researcher at Aix-Marseille University in France who discovered it.

JEAN-MICHEL CLAVERIE: They don't have a regular shape, like regular viruses. They really look like blobs.

BRUMFIEL: Claverie first stumbled across giant viruses a decade ago, when another researcher brought him one that was misidentified as bacteria. This time, he and his team went on the hunt for big viruses. They'd seen hints that they might live underwater, so they teamed up with oceanographers and scooped out sediment samples from the coast of Chile and a freshwater pond in Australia.

CLAVERIE: We bring back the sample and we put the sample in contact with our laboratory amoebas.

BRUMFIEL: Amoebas are microscopic critters and the team used them to tell if there are giant viruses in a sample. Amoebas eat the viruses and get sick.

CHANTAL ABERGEL: If they die, we suspect that there's something in there that killed them.

BRUMFIEL: Chantal Abergel is Claverie's co-author. She's also his wife.

ABERGEL: And it worked.

BRUMFIEL: The amoebas died and the team found that they had two kinds of new Pandoravirus. When they sequenced the genomes of the new viruses, they got a shock. The genetic code is about twice as big as any virus found to date and it seems almost completely unlike anything else on the planet. Claverie says he thinks the Pandoravirus may have started as an ancient form of life.

CLAVERIE: We believe that those new Pandoraviruses have emerged from an ancestral cellular type that no longer exists.

BRUMFIEL: Eugene Koonin, who wasn't involved with the research, isn't buying this theory.

KOONIN: These viruses, unusual as they might be, are still related to other much smaller viruses.

BRUMFIEL: Koonin thinks that the Pandoravirus' bulky genome may be a mishmash of random genetic material it sucks up from the things it's infected. He says the hunt for more giant viruses is only just beginning.

KOONIN: We are going to see many, many more giant viruses discovered around the world, some of which probably will be bigger than Pandoraviruses.

BRUMFIEL: Meanwhile, Claverie is looking at what Pandoravirus actually does in the wild. The fact that it can be found in Australia and Chile, in fresh water and salt, suggests it may be a big player in underwater ecosystems around the globe. Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.